1. Field of the Invention
This invention relates in general to scanning and reproducing means such as for reproducing color and black and white patterns.
2. Description of the Prior Art
In reproduction processes for a black and white scanner used for obtaining from an original picture a reproduction which has been corrected and adapted for subsequent printing processes, it is necessary to convert the original picture into electrical information and subsequently form the reproduction from the information thus obtained.
For obtaining the electrical signal from an original picture, the picture is clamped onto a rotating scanning drum which is scanned dot-wise and line-wise by a light beam and depending on whether reflective original or a transparent original is to be produced the light is reflected or transmitted through the original picture and passes into a scanning instrument which is moved along parallel to the scanning drum and is converted electro-optically in the scanning device into the picture image signal. The picture image signal represents the tonal values of the scanned picture image dots. The entire tonal value scale is limited by the tonal values "white" and "black". By scanning a white spot of the original picture, the so-called white point dot which is the maximum picture image signal is generated whereas by scanning the blackest picture spot or point a so-called black point dot minimum image picture signal is generated.
The picture image signal is supplied into a gradiation stage in which a basic gradition which is dependent on the type of printing is accomplished and the printing process and the printing carrier are selected and the gradiation is varied for desired editorial changes in the redproduced picture relative to the original picture.
The image picture signal which is modified in the gradiation stage modulates the brightness of a recording lamp as the reproduction is made with the recording instrument. The recording carrier may be in the form of a film which is mounted on a recording drum which is also rotated and is exposed dot and line-wise to the recording lamp which is moved axially along the recording drum. The exposed and developed film is used for the reproduction of the printing picture or form.
The tonal values "white" and "black" are fixed end values on the gradiation curve or scale and for changing the tonal value curve of the reproduction from the original only the path of the curve between the end values "white" and "black" is changed. So as to establish these end values, normal voltage levels of the picture image signals are assigned to the tonal values "white" and "black" and such levels are designated as white level for the tonal value "white" and "black" lever for the tonal value "black". Since, however, the original pictures have different brightnesses and contrasts, different picture image signal voltages are derived during scanning of the white dots or, respectively, of black dots of the various picture originals. It is an object, therefore, to balance the picture image signal voltage to the white level or, respectively, to the black level before reproduction of an original picture.
On conventional scanners, the balancing or equalization is accomplished with the aid of potentiometers which are manually adjusted. For this purpose, a white dot of the original picture is placed under the scanning instrument and the picture image signal voltage is adjusted to the white level by manually rotating the potentiometer using a volt meter until the desired level is reached. Thereafter, a black dot of the original picture is scanned and the picture image signal voltage is adjusted to the desired black level by adjustment of an additional potentiometer by manually changing its settings.
These manual adjustments must be repeatedly checked and repeated since precise equalization and balancing can only be done approximately with the potentiometers which inter-relate to each other such that their influence effects each other.
A color scanner is used in the reproduction of a color picture original in order to produce corrected color separated signals from the picture original to be produced and the color separated signals in turn serve for producing printing forms of a color set for multicolor printing.
In a color scanner, the light reflected or transmitted by the color picture original is split up in the scanning instrument into three partial beams which are fed to different color channels. Color channels for color separation and electro-optical converters for obtaining three color signals are associated with the color channels with the three color signals representing the color portions of the scanned image dots such that the logarithmated color signals are proportional to the color densities.
For the purpose of color correction, the color signals are fed to a color computer which has an output comprising three corrected color separation signals for recording the color separations "Magenta" "Cyan" and "Yellow" are obtained. A color separation switch allows the color separated signals to be obtained one after another and supplied through a gradiation stage to a recording lamp whose brightness is modulated by the selected color separation signal. The recording of the color separations occurs by means of the dot and line-wise exposure of a film by means of the recording lamp. The exposed and developed film is the desired color separation.
In a color scanner before the recording of the color separations, the white level and the black level must be individually adjusted for each original picture to be reproduced. Additionally, the varying sensitivity of the individual color channels in the electro-optical conversion of the instant light into the color signal voltages must be taken into consideration.
It is a prerequisite for true-color reproduction that for the same intensity of the partial beams the color channels produces equal color signal voltages. Because of the differing characteristics of the color filters, electro-optical converters and amplifiers, this prerequisite is not met by systems of the prior art.
The partial beams of equal intensity are obtained during scanning of a neutral color picture spot which could be represented by way of example by a white dot, a black dot or a gray dot or surface. True color reproduction occurs then, for example, if a white part of the reproduced picture is also reproduced as a white tonal value without color cast. This condition is achieved when the color signal voltages for all color channels during the scanning of a neutral color portion of the picture are equalized or balanced to equal levels which preferably is the "white" level. For adjusting the color signal levels of the white dot and the black dot and for equalizing and balancing the sensitivity of the different color channels potentiometers are provided in color scanners of the prior art but such potentiometers influence each other as individual ones are adjusted and therefore make it necessary to periodically make adjustments for color correction.
In normalizing the color signal voltages, two different cases can be differentiated.
Number one, the color picture original has white and black portions, in other words, includes white and black dots.
In this case, all color channels' color signal voltages obtained by scanning of the white dot portion must be adjusted to the white level and the color signal voltages obtained by scanning the black dot portion must be adjusted to the black level. By normalizing all of the color signal voltages to the white level, special equalizing or balancing of the color channels to equal sensitivity is eliminated.
For carrying out this objective, the operator must separately measure each of the color signal voltages with a volt meter for each color channel and must then equalize or balance ach channel by rotating the control knobs of the different potentiometers.
Condition two is where the color picture original includes no white or black portions. In such a case, the lightest portion of the picture is selected to represent the "fictitious white dot" and the darkest part of the portion is selected to represent the "fictitious black dot". The fictitious white and black dots differ from pure white or black dots by color cast.
If this color cast is to be eliminated during reproduction, for example, the lightest or brightest color portion of the picture is to be reproduced as purely white or black area of the picture, then the varying color signal voltages obtained during the scanning of the, for example, fictitious white dot must be normalized to the white level and the color signal voltages obtained during scanning of the fictitious black dot must also be normalized to the black level. The objectives under case two, are then the same as those that exist under the first condition described above.
If, on the other hand, the original picture is to be reproduced true to color, then the following process must be accomplished.
First, the previously described equalization and balancing of the differing sensitivities of the color channels must be accomplished wherein a neutral color gray surface is scanned and the color signal voltages obtained therefrom are equalized or balanced to the same level preferably to the white level by changing the gains of the individual color channels.
Then the fictitious white dot of the original picture is scanned while maintaining the previously adjusted gain rates and three different color signal voltages are generated depending on the color composition of the fictitious white dot with one of the color signal voltages maintained at a maximum level. The maximum color signal voltage is raised to the white level. For a true color reproduction, however, the relationship between the three color signal voltages representing an image dot must also be maintained so that after the adjustment of the maximum color signal voltage to the white level, the other two color signal voltages are also raised proportionally by a correctional factor which is equal to the quotient of the white level and the maximum value.
Subsequently, adjustment of the color signal levels for the black dot scanning is accomplished.
For adjusting the color signal levels of the white dot, the operator thus initially has to measure all of the color signals voltages with a volt meter and determine the maximum value and calculate the correctional factors. Then with the aid of the calculated correctional factors, he must determine the values to be adjusted and must then set such values by the rotation of the control knobs of the potentiometers. Since the potentiometers for equalizing and balancing to the white level, and the black level, each influence each other the adjustments must be repeatedly checked and, if necessary, changed which requires a substantial amount of time for such control.
For economical utilization of scanning devices, however, it is necessary and desirable that the adjustment time for a reproduction process be as short as possible relative to the actual reproduction time. Thus, shorter adjustment times can only be accomplished if as many adjustments as possible are automatically accomplished.
Attempts have previously been made to replace the manually controlled potentiometers necessary for the level balancing and equilization with motor control potentiometers. The use of such motor control potentiometers, however, does not result in success since the time for setting the potentiometers with a motor is not substantially less than that using the conventional methods. This is particularly true because the setting must be made with a high degree of accuracy.